A target polypeptide can be prepared by expressing a fusion protein including the target polypeptide and a protein tag that improves protein expression and facilitates protein purification. The protein tag can be removed by protease cleavage. A target polypeptide thus prepared, however, may be inactive due to possible addition of terminal amino acid residues.
Different expression systems might be needed for preparation of different target polypeptides. For example, glycopolypeptides are preferred to be expressed in mammalian cells, which possess glycosylation machinery. As another example, polypeptides toxic to mammalian cells can only be expressed in non-mammalian expression systems. Therefore, a single expression system is not sufficient for producing a large variety of target polypeptides.